Medullary carcinoma of the thyroid is an aggressive neoplasm of the parafollicular cells which results in excessive secretion of thyrocalcitonin. The objective of the study is to devise a reliable, comprehensive plan to follow a kindred of 127 patients, half of whom are at risk to develop medullary carcinoma of the thyroid, in an effort to identify patients with premalignant lesions. These patients will be followed with calcium infusion tests, urinary catecholamines and physical examinations.